1. Field of the Invention
The invention relates to an improved capacitance humidity sensing element for use in humidity measuring and control systems. More particularly, the invention relates to an improved capacitive moisture detector having greatly enhanced sensitivity and to a method of making such a detector.
2. Brief Description of the Related Art
Humidity can be measured by a number of techniques which are based upon the reversible water absorption characteristics of polymeric materials. The absorption of water causes a number of physical changes in the polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer.
Two of the most common physical changes are the change in resistivity and the change in dielectric constant which can be respectively translated into a resistance change and a capacitance change.
Arrangements utilizing the resistance change may, for example, be constructed in accordance with U.S. Pat. No. 3,559,456 to F. Lomker et al.
It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result is erroneous readings.
Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. However, it is important in the construction of capacitive elements to avoid the problems which can arise with certain constructions for such elements.
Exemplary of the capacitive type element is the element shown in U.S. Pat. No. 3,802,268 to Paul E. Thoma.
In the '268 patent a sheet of cellulose acetate butyrate is sandwiched between two planar electrodes, one of which is porous to allow water molecules to equilibrate with the bulk of the film. Among the problems which are encountered with this type of construction is the slow response due to the thickness required to support the structure. There is also a difficulty in fabricating a conductive yet porous electrode.
In addition, there is also inaccuracy incurred at high relative humidity values in that the high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element.
By making the component parts of the element thin, the above mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content of air over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables.
Humidity sensing elements of the capacitance sensing type usually include a moisture insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, highly moisture sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and reaching equilibrium in a short period of time.
A well regarded humidity sensing element of this type is disclosed in U.S. Pat. No. 4,429,343 issued to Freud and illustrated in prior art FIGS. 1-4.
Referring now to prior art FIG. 1, the humidity sensing element includes a planar non-conducting borosilicate glass substrate 10 with a thin metal film electrode system deposited thereon and a dielectric polymer coating 22.
The electrode system includes a first set of fingers 12 which are located in an interdigitated configuration with a second set of fingers 14. The fingers 12 are all connected in parallel to a common bus 16 which is in turn connected to the contact structure 18 at which point electrical contact is made with the measuring instruments to be used.
The other set of fingers 14 are connected in parallel to the bus 19 which is in turn connected to the contact 20, for which electrical connection is provided to the measuring instrument.
The interdigitated fingers are more clearly shown in the enlarged portion of the element shown in prior art FIG. 2 where it is shown that the distance from the center of a finger of one set to the center of the next finger of that set is identified as the period P.
The capacitance of this structure depends on the dielectric constant of the polymer coating. This dielectric constant changes depending on the amount of water vapor present in the coating. The amount of water vapor present in the coating depends on the water vapor partial pressure of the atmosphere ambient the coating and the water vapor partial pressure is proportional to relative humidity.
The structure of the element of prior art FIG. 1 and its relationship to the dielectric polymer coating 22 which overlays the fingers is shown in more detail in prior art FIG. 3 where the polymer 22 is shown overlaying fingers 12 and 14 which are shown as being deposited on the non-conducting substrate 10.
As shown in prior art FIG. 3, the thickness of the polymer coating is greater than the period of the fingers in order to minimize the effects of surface contamination. For example, if oil or grease contacts the surface of the coating, there will be a change in capacitance which is unrelated to relative humidity.
Since the capacitance between the sets of interdigitated fingers is determined by the weighted average of the dielectric constant of the polymer coating, that portion of the coating closest to the surface of the fingers must be weighted the most and that portion furthest from the fingers the least.
If the coating is thick enough there will be portions of the coating, at its surface and away from the fingers, which will be a sufficient distance from the surface of the fingers so as to have a negligible effect on the average dielectric constant. Thus, if the coating is maintained thick enough to place the surface far enough from the finger surface, the influence of contaminants on the surface should be negligible.
Devices of the type shown in prior art FIGS. 1-3 have been utilized to measure the relative humidity to an accuracy of 1% and a stability at 95% relative humidity of better than 1% in one month.
The solution presented in the '343 patent is based on the fact that the field used to make the capacitance measurement extends a certain distance normal to the electrodes. If the field extends beyond the surface of the polymer, contaminants on the surface of the polymer will affect the field. The problem with making the dielectric coating so thick that it captures all of the electric field is that the device becomes very slow in detecting changes in relative humidity.
Another approach to this problem is to make the electrodes and the period P smaller so that a relatively thin film dielectric could be used. This approach is limited by the resolution of the deposition technology used to place the electrodes on the substrate.
Another effective approach to the problem of surface contamination is illustrated in prior art FIG. 4.
According to this solution, a substrate 10' is printed with electrodes 12', 14' in the usual manner. A first thin film of dielectric polymer 22' is coated over the electrodes. A very thin porous conductor 23 is placed over the thin film 22' and a second thin polymer film 22" is coated over the conductor 23.
The theory behind this solution is that the conductor 23 captures most of the electric field and the second film 22" captures any of the field which manages to escape the conductor 23.
While this approach has merit, it does not render the detector any more sensitive than the other structures described above. In fact, the conductive layer 23 changes the circuit of the sensor to be two capacitors in series rather than a single capacitor. This reduces the capacitance by a factor of two and thus decreases the sensitivity of the detector.
Modern manufacturing processes require measurement of moisture contents corresponding to dew points below -40 degrees C or a relative humidity of less than 0.1%.
There is a need for a durable, compact, efficient moisture detector which can be used effectively in these processes to measure very small moisture content in gaseous atmospheres.